1. Field of the Invention
The present invention relates to a photocurable resin composition which inhibits the deactivation of a photopolymerization initiator and is suitable for coating materials for plastics, wood, ceramics, glasses, metals, paper, etc., photo-fabricated materials, three-dimensional moulding materials, printing plate materials and the like, particularly suitable for a coating material for optical fibers, a method for producing the resin composition, and an optical fiber.
2. Description of the Related Art
Glass fibers, which are utilized for an optical fiber, are very fragile and subjected to flaws. Besides, they suffer from a greater light transmission loss when contaminated. Therefore, glass fibers are protected and reinforced by a measure which comprises coating the surface of the glass fibers with a urethane (meth)acrylate-series ultraviolet ray-curable resin (hereinafter, it may be referred to simply as a UV resin) immediately after the drawing of the glass fibers (primary coating), and then coating the primary-coated fibers with an ultraviolet ray-curable resin (secondary coating), the former resin having a low Young's modulus as well as being soft and less dependent on temperatures, whereas the latter resin having a high Young's modulus. For identification of optical fibers, they may be sometimes coated with a coloured material (tertiary coating). A piece of fiber thus coated, including the one coated with the tertiary coating, is called an optical fiber elemental strand. The material for bundling the optical fiber elemental strands and making them into such a tape-like structure is called a bundling material (or a taping material). The UV resin is also used as the taping material. An optical cable is constituted with a bunch of several tape core-strands contained in a sheath.
An optical fiber to be led into general homes is called a fiber to the home or a drop wire, which is made of a wire comprising one or a few fiber elemental strands thickly coated with a UV resin having a high Young's modulus.
There are some common characteristics required of these UV-coating resins: to have an appropriate viscosity, to show little changes in viscosity during a long storage, to be rapidly curable and capable of being cured (hardened) even with a low-quantity ultraviolet ray irradiation, to generate only a small amount of hydrogen gas, to ensure long-term reliability of a coated glass fiber, etc.
Recently, the drawing speed of a glass fiber from a molten glass base can be speeded up for the purpose of improving the productivity of optical fibers. However, the high-speed drawing causes to give a less quantity of ultraviolet ray irradiation during the next UV resin-coating process, and ends in giving a cured product (hardened product) with a low Young's modulus. Therefore, there have been suggested resin compositions, as the photopolymerization initiator, which comprises a highly active and quickly curable acylphosphine oxide-series photopolymerization initiator. For example, in regard to a composition comprising a monoacylphosphine oxide, Japanese Patent Application Laid-open Nos. 6125/1992 (JP-A-4-6125) and 296315/1992 (JP-A-4-296315) disclose a photocurable resin composition containing a photopolymerization initiator which comprises a mixture of polyurethane having ethylenically unsaturated bonds, a monomer having an ethylenically unsaturated bond, an acylphosphine oxide, and a compound having a tertiary amino group (a compound having a morpholine ring). These literatures disclose the use of a tin compound as a urethanating catalyst.
As to a composition comprising a bisacylphosphine oxide, Japanese Patent Application Laid-open No. 298818/1994 (JP-A-6-298818) corresponding to U.S. Pat. No. 5,534,559 teaches a photocurable resin composition comprising an ethylenically unsaturated monomer and a bisacylphosphine oxide (e.g. bis(2,4,6-trimethylbenzoyl)-2-methylpropylphosphine oxide).
Japanese Patent Application Laid-open No. 259642/1996 (JP-A-8-259642) discloses a photocurable resin composition comprising a polymer having an ethylenically unsaturated bond such as urethane acrylate, a monomer having an ethylenically unsaturated bond, and a bisacylphosphine oxide (e.g. bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide) which shows a higher curability than a monoacylphosphine oxide. A tin compound is used as urethanating catalysts in this literature.
Further, Japanese Patent Application Laid-open No. 127630/1996 (JP-A-8-127630) discloses a photocurable resin composition comprising a urethane (meth)acrylate, N-vinylpyrrolidone, a tertiary amine and a photopolymerization initiator (e.g. 2,4,6-trimethylbenzoyl diphenylphosphine oxide). In this literature, too, a tin compound is used as a urethanating catalyst.
Incidentally, Japanese Patent Application Laid-open No. 306146/1993 (JP-A-5-306146) corresponding to U.S. Pat. Nos. 5,527,835, 5,538,791 and 5,587,403 discloses a coating composition comprising a polyether polyolseries urethane oligomer prepared with the use of an aliphatic polyisocyanate, a reactive diluent, a silane compound, and a conventional photopolymerization initiator. This literature suggests a tin compound, a metal salt of an organic acid, an amine, or the like, as the urethanating catalyst.
A resin composition containing an acylphosphine oxide as a photopolymerization initiator has a high photocurability. However, the acylphosphine oxide is deactivated during the storage of the resin composition, and, accordingly, its photocurability is deteriorated to a great extent. Especially in the presence of water, no matter how little the quantity may be, the acylphosphine oxide is deactivated in a short period of time, and, in an extreme case, it completely ceases to act as a coating composition. The deactivation of the photopolymerization initiator may be prevented by excluding water from the resin composition. But it is practically impossible to keep the water content (moisture content) at zero in the resin composition.
In another aspect, the generation of hydrogen gas causes an increase in light transmission loss. Since hydrogen gas has a high diffusion coefficient in resins and glasses, hydrogen diffuses throughout a quartz fiber even under normal conditions for use. The diffused hydrogen is trapped at a defect site of the main components of the quartz fiber, such as SiO.sub.2 and a dopant GeO.sub.2, whereby, it is assumed, the loss in light transmission increases. Taking this defect into consideration, an ultraviolet ray-curable resin composition for optical fiber coating is desired to give out hydrogen gas in a small quantity. By way of illustration, a coating material for glass fibers is disclosed in Japanese Patent Application Laid-open No. 11611/1990 (JP-A-2-11611), which is prepared by adding a phenolic or sulfuric antioxidant to a composition comprising a compound having an ethylenically unsaturated bond such as a urethane acrylate and a compound having a piperidine ring-containing (meth)acrylate.
This resin composition is effective in reducing the amount of the hydrogen gas generation. However, the addition of the antioxidant causes deteriorations of the curing rate or the gel fraction. Therefore, it is difficult to reduce the generation of hydrogen gas, and to retain the high characteristics of the photocurable resin composition, at the same time.
Japanese Patent Application Laid-open No. 77514/1992 (JP-A-4-77514) teaches that a polyurethane acrylate prepared by using a hydrogenated dimerdiol obtained by hydrogenating a C.sub.36 dimeric acid shows a high adhesive property to a base having a low surface tension (e.g. the printing surface of an offset ink, polypropylene), and that the cured layer has a low water-absorbability. Japanese Patent Application Laid-open No. 262848/1993 (JP-A-5-262848) corresponding to U.S. Pat. No. 5,416,880 describes that a polyurethane (meth)acrylate based on a polyester polyol having a residue of an optionally hydrogenated dimeric acid exhibits a low water-absorbability and excellent resistance to hot water. Japanese Patent Application Laid-open No. 310545/1992 (JP-A-4-310545) discloses that the secondary coating material and the taping material having a high Young's modulus and showing an elongation property can be made of a polyurethane (meth)acrylate prepared by using a compound with an average molecular weight of 500 or less which contains an OH group at the terminal and no polymerizable vinyl group (e.g. polyhydric alcohols such as 1,10-decanediol, 4,4'-methylenebiscyclohexanol).